(1) Field of the Invention
The present invention relates to an apparatus for preparing a heat-set plastic hollow vessel. More particularly, the present invention relates to an apparatus for preparing a heat-set plastic hollow vessel, which is biaxially molecularly oriented by drawing, in one mold at a high efficiency.
(2) Description of the Related Art
A biaxially draw-blow-formed vessel of a thermoplastic polyester such as polyethylene terephthalate (PET) has a high transparency and an excellent surface gloss and also has high impact strength, rigidity and gas barrier property required for a bottle. Accordingly, vessels of this type are utilized for bottling various liquids.
However, polyester vessels are defective in that the heat resistance is poor, and when the content is hotfilled, thermal deformation or change of the volume by shrinkage occurs. Accordingly, various proposals have been made for heat-setting a biaxially draw-blowformed vessel after the forming operation.
The heat-setting method is roughly divided into a so-called one-mold method in which the draw-blow forming and heat-setting are carried out in one common mold and a so-called two-mold method in which the draw-blow forming and heat-setting are carried out in different molds.
According to the one-mold method, as disclosed in Japanese Patent Publication No. 6216/84, the draw-blow forming and heat setting are simultaneously carried out in a blow-forming mold, and a relatively long residence time in the mold is necessary for effecting the heat-setting after the draw-blowing operation and the cooling for the withdrawal of a formed hollow vessel and therefore, the manufacturing speed is still low. As the one-mold method, Japanese Patent Application Laid-Open Specification No. 95666/79 proposes a process in which the temperature of the hollow mold is maintained at a highest level within the range where a final hollow formed body can be withdrawn without deformation under no substantial cooling, for example, at 100.degree. C., and high-temperature high-pressure air is blown in a polyester preform and simultaneously, the preform is biaxially drawn. According to this process, the temperature of the mold need not be elevated or dropped, but the heat-setting is effected by heat transferred from the high temperature gas and a relatively long time is necessary for completion of the heat-setting because of the presence of the heat transfer interface. Moreover, the attained resistance to thermal shrinkage is still insufficient.
As the two-mold method, there are known a process in which a formed body obtained by the draw-blow forming disclosed in Japanese Patent Publication No. 56606/85, and a process in which the draw-blow forming and the heat treatment are simultaneously carried out in a primary mold and the formed body is taken out from the primary mold and is blow-formed again in a secondary mold without cooling, as disclosed in Japanese Patent Application Laid-Open Specification No. 53326/82.
However, the two-mold method is defective in that two molds are necessary for the forming and heat-setting operations, the apparatus cost is increased and the number of the manufacturing steps is increased. Therefore, further improvements are desired.